1. Field of the Invention
The present invention relates to a process for producing an aerogel. More specifically, the present invention relates to a process for producing a silica aerogel, which has properties such as heat insulating properties peculiar to porous materials. Additional properties of the aerogel include and transparency, as well as excellent hydrophobic nature.
2. Description of the Prior Art
A process for producing an aerogel which is useful as a translucent heat insulator has been disclosed in U.S. Pat. No. 4,402,927, U.S. Pat. No. 4,432,956, and U.S. Pat. No. 4,610,863. These patents disclosed a following process for obtaining an aerogel. That is, hydrolysis and condensation polymerization of alkoxysilane (also named silicon alkoxide or alkyl silicate) are carried out, and then the resulting gel is dried in the presence of a dispersion medium under a supercritical condition over a critical point of the dispersion medium (such a drying method is called supercritical drying), to obtain the aerogel.
In this process, the dispersion medium used in supercritical drying is carbon dioxide or organic solvents such as ethanol and the like. However, in the case of using carbon dioxide as the dispersion medium, since the resulting aerogel retains silanol groups on the surface therefore, it manifests hygroscopicity and therefore adsorbs moisture in the atmosphere. Further, silanol groups having high hydrophilicity are formed on the surface of the aerogel by adsorbed water, which adsorbs more moisture in the atmosphere over time. On the other hand, in the case of using organic solvents such as ethanol and the like as the dispersion medium of supercritical drying, since the resulting aerogel retains alkoxyl groups on the surface, the aerogel retains hydrophobicity immediately after the supercritical drying. But, the alkoxyl groups are hydrolyzed by moisture in the atmosphere over time, so that silanol groups are formed on the surface of the aerogel and, consequently adsorbs moisture in the atmosphere in the same way as the above case.
Therefore, there is a problem that optical properties such as transparency or thermal properties such as heat insulating properties of the aerogel are lowered, and dimensional change or cracks arises from shrinkage caused by moisture adsorption, so that deterioration of quality or performance arises, resulting in lack of practical use.
The present inventors have proposed a process for producing an aerogel, which can prevent properties therefore from deteriorating over time by imparting hydrophobicity thereto, in JPA-HEI 5-279,011 and JPA-HEI 7-138,375. In these processes, alkoxysilane compounds are used as a starting material and an aerogel is produced through the steps of hydropbobilizing treatment and supercritical drying.
However, in a process for producing an aerogel using alkoxysilane, there is the following problem. In the process, where alkoxysilane is hydrolyzed in an organic solvent such as an alcohol with water and a catalyst, the resulting product is condensation-polymerized, and a gel is prepared through a sol state; but, in this case, since organic solvents such as alcohol used as a solvent have relatively high volatility, when the gel obtained is exposed to the air, the organic solvents contained in the gel is volatilized in a short time, so that the resulting aerogel is liable to manifest cracks or shrinkage before supercritical drying. This particularly occurs in the steps of preparation of the sol, gelation and curing of the gel. Therefore, there is a problem that the resulting aerogels obtained after supercritical drying include some off-qualities such as cracks and the like, and the yield of non-detectives thereof becomes lower.
In order to prevent an organic solvent of an alcohol and the like used in the sol or the gel from volatilizing, it is necessary to attain the airtight condition or to fill the atmosphere with the vapor of the same organic solvent from the time point of preparing the sol. Curing the gel is effective for strengthening the gel structure to heat at a range from room temperature to several tens of .degree. C. However, it is not preferable from a safety viewpoint to heat under the condition of filling the atmosphere with the vapor of an organic solvent.
In view of these circumstances, the present inventors investigated starting materials as alternatives alkoxysilane and paid attention to water glass as a promising material. Another process for producing an aerogel made from water glass is disclosed in JPA-SHO47-15,398, U.S. Pat. No. 5,137,927 and JPA-HEI2-304,299. In the process, since supercritical drying is performed using an alcohol as a dispersion medium, the resulting aerogel has transient hydrophobicity, but deteriorates over time, as described above.